1. Field of the Invention
The present invention is directed to a method for the operation of a magnetic resonance apparatus.
2. Description of the Prior Art
Magnetic resonance technology is a known technique for generating images of a body interior of an examination subject. To that end, rapidly switched gradient fields are superimposed on a static, basic magnetic field in a magnetic resonance apparatus. Further, radio-frequency signals are radiated into the examination subject for triggering magnetic resonance signals. The resulting magnetic resonance signals are registered and image datasets and magnetic resonance images are produced on the basis thereof. The magnetic resonance signals are deleted (received) by a radio-frequency system, demodulated in phase-sensitive fashion and converted into values respectively represented as complex numbers by sampling and analog-to-digital conversion. These values are entered into a k-space matrix forming an image dataset. Using a multi-dimensional Fourier transformation, an appertaining magnetic resonance image can be reconstructed from the k-space matrix occupied with these values.
In a known method for magnetic resonance imaging, at least one-half of the locations in the k-space matrix is occupied with values that, as described above, are acquired directly from magnetic resonance signals. The other half of the locations is occupied with values that are respectively calculated from the aforementioned values by complex conjugation. It is thereby assumed that the respective values of two locations of the k-space matrix that are point-symmetrically arranged with respect to a symmetry point of the k-space matrix, generally a selected zero point, behave in conjugate complex fashion relative to one another. Methods of this type are known as half-Fourier techniques and are disclosed in greater detail in, for example, U.S. Pat. No. 5,043,665.
Without countermeasures, positional changes of a region of the examination subject relative to the magnetic resonance apparatus during an overall time span of the registration of the k-space matrix lead to unwanted distortions of the magnetic resonance image. Such positional changes can arise, for example, due to movement of the examination subject, for example a patient.
The article by H. Eviatar et al., xe2x80x9cReal Time Head Motion Correction for Functional MRIxe2x80x9d, Proc. of ISMRM 7 (1999), page 269 discloses a method wherein positional changes of a patient""s head during the overall registration time of an image dataset are acquired in the framework of a functional magnetic resonance imaging, and wherein an acquired positional change is taken into consideration during the further registration of the image dataset. To this end, the magnetic resonance apparatus has an optical acquisition system with which optical reflectors attached to the patient""s head can be monitored as to their position.
Further, positional changes of the region to be imaged during a registration of diffusion magnetic resonance images are especially critical. For producing a diffusion image, at least one first image dataset and one second image dataset of the region to be imaged are registered, the first being registered, for example, with a sequence having diffusion-emphasizing gradient pulse with high strength and a long duration, and the second being registered without the aforementioned gradient pulses. The diffusion image derives from a corresponding subtraction of the two image datasets in the display thereof. In particular, positional changes of the imaged region during the registration of the diffusion-emphasizing image dataset lead to serious misinformation with respect to the diffusion to be actually acquired. An identification of positional changes is therefore implemented when registering the diffusion-emphasizing image dataset with a sequence that has a number of radio-frequency excitation pulses with temporally intervening acquisition phases. Effects of positional changes that are identified are thereby correspondingly corrected. Similar to navigator echo technique, a so-called correction echo is generated and registered at every radio-frequency excitation and is compared to a reference correction echo for acquiring the positional change. Further details of this approach are described, for example, in the article by R. J. Ordidge et al., xe2x80x9cCorrection of Motional Artifacts in Diffusion Weighted MR-images Using Navigator Echosxe2x80x9d, Magnetic Resonance in Medicine (12), 1994, pages 455-460. The overall registration time of the diffusion image is lengthened due to the correction echos.
An object of the present invention is to provide a method for the operation of a magnetic resonance apparatus with which positional changes of an imaged region during an overall registration time of a k-space matrix can be determined in a simple and time-efficient way, so that image artifacts arising therefrom can be correspondingly corrected.
This object is inventively achieved in a method for the operation of a magnetic resonance apparatus wherein nuclear magnetic resonance signals obtained from an image region of an examination subject that is positioned in an imaging volume of the apparatus are entered into a k-space matrix as respective complex numbers are a motion model is determined with which translational motion of the subject with respect to the imaging volume can be described with a time-dependency, at least for prescribable points of the image region. The motion model is determined with respective phase values of at least two values of the k-space matrix that are point-symmetrically located in the k-space matrix with respect to a symmetry point of the k-space matrix. These two values behave as complex conjugates of each other in the absence of any motion between the registration times of the two values, and, any translational motion is thus reflected in the phase values. The values of the k-space matrix are corrected according to the motion model determined in this manner.
As a result, motion of the examination subject which occurs during registration of an image dataset can be identified and the effects thereof on the magnetic resonance image can be corrected without correction or navigator echos that lengthen the exposure time having to be registered for this purpose, and without having to provide any kind of additional motion acquisition devices, such as optical devices.
The inventive method is not limited to k-space matrices that are completely occupied with values acquired from magnetic resonance signals. The method can likewise be applied to k-space matrices wherein only a little more than a half of the matrix locations are occupied with values acquired from magnetic resonance signals. The method thus also can be employed in the half-Fourier techniques wherein generally more than half of the values are directly acquired. The acquired values, exceeding half of the matrix locations, are correspondingly utilized for determining the motion model.
An especially advantageous employment of the method is for a magnetic resonance image exposure of shoulders of a patient. As experience has shown, it is difficult for the patient to keep the shoulders still for several minutes in a designated fixed position required for the exposure.
In a preferred development, the time-dependency of the translational motion is described by registration times for values of the k-space matrix. Given knowledge of the registration times of all values of an image dataset, any translational motion can be identified and correspondingly corrected regardless of whether it occurs during an acquisition phase for magnetic resonance signals or in preparation phases between the acquisition phases.
In another embodiment, motion between registration times of values that are to be allocated to a single radio-frequency excitation is left out of consideration. The determination of the motion model thus is correspondingly simplified. This is particularly meaningful given sequences wherein longer preparation phases occur between short acquisition phases for the registration of the k-space matrix wherein magnetic resonance signals are acquired. Such preparation phases are required for the preparation of a spin ensemble in order to prepare corresponding magnetic resonance signals. This is typical of spin echo sequences.
In a further embodiment, the registration of, for example, the values for filling the k-space matrix is implemented with a slice technique. The resulting k-space matrix thus is two-dimensionally formed. The k-space matrix can then be considered a k-plane with a symmetry point. The same applies to the individual slices of a three-dimensional image dataset for which values are registered with a multi-slice technique. In multi-slice technique, a three-dimensional image dataset is generated that contains a number of two-dimensional k-space matrices.
In another embodiment, the registration of the values for filling the k-space matrix occupied is implemented with a volume technique. In contrast to the slice technique, a slice selection gradient is replaced by an additional phase coding gradient in the volume technique, so that a three-dimensional k-space matrix is registered.